One of the factors for providing a comfortable indoor climate is control of humidity. While humidity does not have to be controlled with the accuracy with which temperature is controlled, it is still important that it be kept within the range of approximately 30-70% RH to prevent various problems. For example, if the relative humidity falls below 30%, furniture in the controlled space may deteriorate and some people have respiratory and skin problems. If the relative humidity rises to above 70%, the space seems clammy and uncomfortable, and there is the possibility of mildew.
Where humidity must be added to the air supplied to the space to raise the humidity to within the 30-70% range, it is also important that the duct humidity be held below the dew point, or 100% RH. If the humidity reaches this level, then condensation can occur in the duct which will cause rust and potentially require premature replacement. Even more importantly, high duct humidity can result in growth of microorganisms which can cause potentially dangerous diseases for the occupants of the space. To prevent excessive duct humidity, it has become standard practice to include a humidity sensor in the duct, and use its output to limit duct humidity to below the dew point.
In the past, space humidity has been controlled in much the same manner as has space temperature. A humidity sensor in the controlled space provides feedback to the humidity control apparatus, and the humidity in the supplied air is then altered so as to shift the space humidity toward the set point value. However, it has been difficult to accurately control humidity in the controlled space with this control algorithm for a number of reasons. These are, in no particular order, that humidity sensors now in use react slowly to changes in humidity, which results in unstable control. The actual humidity of the space does not always change predictably in response to changes in the humidity of the supplied air. And the necessity of limiting duct humidity to less than the dew point affects the operation of control algorithms based on the space humidity.